In scales generally, it is conventional to mount a weigh frame above a base and rigidly connect load cells between the weigh frame and the base. As subject matter to be weighed is placed on the weigh frame, the stressing of the load cells, through the application of the weight, causes the load cells to generate an electrical signal proportional to the weight applied to the load cells. The loads measured by respective load cells are summed to provide the measure of the weight of the subject matter mounted on the scale.
The rigid or substantially rigid connection between the base and the weigh frame is satisfactory where the base is fixed with respect to ground. But where the scale is to be moved from place to place on a regular basis, or where the load is applied in diverse directions or attitudes, thereby introducing force vectors that are not parallel to the force of gravity, the rigid mounting of the weigh frame with respect to the base frame will introduce errors. Any twist of the weigh frame with respect to the base frame will cause binding at the load cell. The frictional force of the binding will be seen as weight that is either added to or subtracted from the true weight, but in any event, will introduce error.
A hospital bed having provision for weighing a patient is an example of the type of scale environment that causes error. This is particularly true in relation to special critical care-type beds that are leased for limited periods, returned, and leased again for use at another site. The frequent movement of the bed from one site to another compounds the problems referred to above. A ground support that is not essentially perpendicular to the force of gravity introduces a twist of the weigh frame with respect to the base frame causing error. The position of the patient--for example, consider a patient in an inclined trendelenburg position--changes the vector of the load on the load cell. Any change from a strictly vertical application of the load, that is, parallel to the force of gravity, introduces error.